This invention has to do with a system of supplying oil to a device to which oil is supplied continuously and which generates substantial heat in varying quantities in the course of utilizing the oil, such as a fluid drive between a steam turbine-generator shaft and a boiler water feed water pump. In this application, the oil is utilized to transfer power in the fluid drive element, to remove the heat generated and also to lubricate the bearings of the fluid drive, the gearbox, if used, and the boiler feed water pump, and to lubricate the flexible gear type couplings, if used.
In a large electric power plant, in which a boiler with the usual tubes supplies steam to a turbine for generating electricity, the boiler feed water pump is a critical element. The flow rate of water to the boiler must be controlled with considerable nicety to match the steam flow requirement of the turbine which varies according to the amount of electrical power being generated at any given moment.
It is customary in many electrical generating plants to use an extension shaft of the main turbine-generator shaft to power the boiler feed pump. That turbine-generator shaft rotates at a constant speed, generally at 3600 revolutions per minute. In order to vary the speed of the feed water pump, hence to vary the water flow to the boiler, a fluid drive is provided between the extension shaft and the boiler feed water pump shaft.
Such a fluid drive consists essentially of an impeller and a runner, both enclosed in a rotating casing, and the whole enclosed in an outer, stationary housing. The impeller and casing are assembled and mounted on the input shaft of the fluid drive, which is coupled to, and driven by, the extension shaft and they rotate at the constant speed of the turbine-generator. The runner is mounted on the output shaft, which drives the boiler feed water pump through a flexible coupling, and if used, a gearbox. The flexible couplings on the input and output ends of the fluid drive may be of any of several designs, depending upon the requirements of the mating shafts, and need not be the same for input and output.
The variable speed capability of a fluid drive is provided by the runner being physically unconnected to the impeller or casing, and by oil within the casing being thrown by the impeller vanes against the vanes of the runner, causing the runner to rotate. The impeller and runner are generally of similar design in that they are made in the form of vanes and pockets interspersed uniformly around 360 degrees, although the numbers of vanes and pockets differ between the impeller and runner. "Circuit" oil is continuously introduced into the impeller/runner/casing and leaves the casing via a scoop tube. The rotation of the impeller and casing causes the oil to move outward, under centrifugal force, to form a vortex or ring, retained on its radially outer side by the inner surface of the casing. The inner radius of the ring of oil is determined by the radial position of the scoop tube within the casing. When the casing is full, that is, when the scoop tube is moved to its radially innermost limit, the slippage between the impeller and runner is low, so that the runner rotates at substantially the same speed as the impeller. In order to reduce the driving forces of the impeller on the runner, the radial thickness of the oil ring is reduced by moving the scoop tube radially outwardly toward the casing. Clearly, if all of the oil were removed, and no oil were being introduced, the runner would remain stationary.
It can be appreciated that the thinner the ring of oil radially, the less efficient is the transmission of energy from the impeller to the oil to the runner, with the energy not transmitted to the runner taking the form of heat generated in the oil within the impeller/runner/casing.
Table 1 below provides a typical representation of the power in, power out, and heat generated (power loss) for a boiler feed water pump of approximately 14,500 hp rated power, wherein the power requirements for the pump are proportional to the cube of the speed, for example. The peak heat generation point occurs when the output speed is approximately 2/3 of the input speed, or at approximately 2400 rpm, for an input speed of 3600 rpm. Depending upon boiler water pressure and flow rates, the fluid drive output speed may be either above, at, or below the speed corresponding to this peak heat generation point.
Also noted in this table are typical power losses related to the bearings and to passing the oil into the impeller and out through the scoop tube. In this process, the oil speeds up from zero to input shaft speed as it enters the impeller, and then is driven into the scoop tube as it exits the casing.
The data of Table 1 are common to the data of Tables 2 and 3, infra. In all three tables, the speed of rotation of the impeller is a constant 3600 rpm.
TABLE 1 __________________________________________________________________________ HP IN HP OUT HP LOSS RPM ROTATING ROTATING ROTATING HP LOSS OUT ELEMENT ELEMENT ELEMENT BEARINGS __________________________________________________________________________ 1. 3510 15,000 14,625 375 200 2. 3300 13,259 12,154 1,105 195 3. 3100 11,700 10,075 1,625 190 4. 2900 10,238 8,248 1,990 185 5. 2700 8,874 6,656 2,218 180 6. 2500 7,609 5,284 2,325 175 7. 2400 7,013 4,675 2,338 173 8. 2300 6,441 4,115 2,326 170 9. 2100 5,369 3,132 2,237 165 10. 1900 4,396 2,320 2,076 160 1700 3,519 1,662 1,857 155 1500 2,738 1,141 1,597 150 1400 2,386 928 1,458 145 800 779 173 606 100 __________________________________________________________________________
There is a continual flow of circuit oil through the fluid drive element (impeller/runner/casing), into the impeller/runner portion, out through the gap between the impeller and runner outer peripheries, into the casing portion and exiting through the scoop tube and through the gap between the impeller and runner outer peripheries, into the casing portion and exiting through the scoop tube and through weep holes in the casing. This circuit oil is discharged through the scoop tube at a velocity of several hundred feet per second, entrains air, and settles as foam in the sump at the bottom of the fluid drive tank.
The lube oil from the bearings in the fluid drive, from the gearbox, if used, and from the bearings of the boiler feed water pump, and from the couplings, if any, also drain into the same sump at the bottom of the fluid drive tank. All of the draining lube oil and discharge circuit oil mixes together in this sump. Clearly, the oil that is used to operate the fluid drive element, i.e., the circuit oil, and the lubricating oil are the same, with the flow paths being separated only in the piping supplying the circuit oil to the impeller/runner/casing element and piping supplying the lube oil to the bearings, gearbox, and couplings.
It is known that the output shaft speed is controlled primarily by the scoop tube position and to some degree by the circuit oil flow rate. In some earlier fluid drives, the speed control was performed entirely by a flow control valve with a series of leakoff orifices in the casing but without a scoop tube.
In conventional arrangements, shown in FIGS. 1, 2, and 3, the lower portion, or sump, of the fluid drive tank is used as an oil reservoir, holding oil in a quantity which provides only about one minute dwell time before entering the pipes leading to the suction of the pumps. This is not enough time to detrain the air from the oil.
Because the sump/reservoir is relatively small, it is very easy to change the level, or as seen by a sight glass, the apparent level, of the oil in the sump/reservoir. The three primary causes of change of oil level are these: (1) The foaming action due to the entrained air; (2) The temperature distribution of the oil throughout the system because oil expands with increasing temperature; and (3) The amount of oil resident in the oil ring in the impeller/runner/casing assembly. Together, all of these influences provide uncertainty as to the precise level of oil in the sump/reservoir, and cause operators to add or remove oil on a daily basis, sometimes on a shift to shift basis.
The cumulative result is that over the years there have been many instances of over filling of fluid drives. This is the single most damaging occurrence, short of a severe mechanical failure such as a broken shaft, that can occur to a fluid drive. When the fluid drive is over filled, the oil enters the open end of the casing through which the scoop tube passes, usually damaging the scoop tube so that it can not accommodate the oil discharge flow, so that oil flow through the impeller/runner/casing is reduced and the power dissipated increases, with the result of very high oil temperatures, on the order of 300.degree. to 400.degree. F. When this occurs, the entire fluid drive and oil system must be dismantled, the coked oil removed, damaged parts replaced, and the entire system rebuilt.
Three arrangements of positive displacement pumps have been commonly used:
Arrangement (1), illustrated in FIG. 1, and designated by the reference numeral 101: Uses one internal oil pump 130 within the reservoir which is driven by gearing from the input shaft, one external A.C. powered oil pump 40, and no D.C. powered oil pump.
Arrangement (2), illustrated in FIG. 2, and designated by the reference numeral 201: Uses three external pumps, two main A.C. powered oil pumps 40 supplying both circuit oil and lube oil, with one emergency D.C. powered lube oil pump 41.
Arrangement (3), illustrated in FIG. 3, and designated by the reference numeral 301: Uses two or, for higher flow requirements, three main A.C. powered circuit oil pumps 40, two main A.C. powered lube oil pumps 140, and one emergency D.C. powered lube oil pump 41.
The oil passes from the pumps through heat exchangers 50, in this case called oil coolers, to remove the heat generated within the element, in the bearings, etc. Control valves are usually used to control the cooling water flow in order to keep the temperature of the oil leaving the coolers as close to the desired set point as possible. The temperature desired in the conventional systems for the fluid drive oil as it exits the cooler is usually on the order of 130.degree. F. For the improved system of this invention, the desired temperature for the fluid drive oil as it exits the cooler is about 110.degree. F., and coolers with a capacity to provide this 110.degree. F. cooler discharge oil temperature are provided. The 110.degree. F. cooler discharge temperature for fluid drive oil then approximates the cooler discharge temperature for the oil for most conventional turbine-generators, thereby eliminating a source of confusion for the power plant operators.
With regard to the circuit cooling oil line feature, many but not all of the conventional oil systems have one.
In present conventional fluid drive oil systems, only the lube oil passes through filters 163, usually two or three parallel filters, so one or two can be valved into service with the other one cut out of service for maintenance.
The lube oil flow rate through the system is constant.
The circuit oil flow rate through the fluid drive element is constant when in service. Arrangements (1) and (2) use a "Trip" valve (shown as TV-2 in FIGS. 1 and 2) that causes the oil flow through the circuit oil piping to go either to the element when the fluid drive is in service or to return to the sump/reservoir in the bottom of the fluid drive tank when the fluid drive is out of service. A "Trip valve" is one that is open or closed, and travels rapidly from one state to the other. In Arrangement (3), FIG. 3, the circuit oil pumps are turned on when the fluid drive is in service, and they are turned off when the fluid drive is out of service.
As shown in Table 1, the heat generated (power loss) in the oil within the element is, in large measure, a function of the relative speeds of the input and output shafts. With a constant flow of circuit oil, the temperature of the circuit oil as it discharges from the element is a similar function of the relative speeds of the input and output shafts, Table 2.
It is highly desirable for the type of oil used in fluid drives, turbines, and similar pieces of rotating machinery not to exceed approximately 185.degree. F., as exceeding this temperature will cause the oil to degenerate, and to form a varnish-like substance which coats all of the oil wetted surfaces. In time, this requires that the oil be replaced, at a minimum. Disassembly and removal of the varnish-like coating often is required, with subsequent reassembly.
In the original design of most of these fluid drives, the intent was to pass quickly through the lower speeds during start-up and to operate entirely in the more efficient range with the output shaft speed above 2700 rpm, usually approaching maximum electrical generation. Consequently, the heat exchangers were designed accordingly; and not for the maximum heat generation point shown in the above example. However, in recent years, it has become necessary to operate many of these electrical generating plants over a very wide range of conditions, including those associated with reduced output shaft speeds causing the operating temperatures to become excessive under some conditions. To reduce the circuit oil discharge temperature, several design changes have been made, including increasing pump size providing greater flow capacity, and adjusting the setting of the pressure relief valve for each pump that pumps oil to the fluid drive circuit. The pressure relief valve is a bypass valve positioned between the output or high pressure side of the pump, and the suction line leading to the inlet or low pressure side of the pump. Operators commonly change the setting of this valve to get more or less flow to the fluid drive. This valve is not intended to have continuous flow, but is intended only for intermittent use to protect the motor and pump in response to abnormally high discharge pressure such as would occur in the event that a downstream blockage happened Nevertheless, in conventional systems, these pressure relief valves often experience continuous flow. Among other things, this decreases the mean time between failures.
The position of the scoop tube is regulated by a controller responsive to the steam flow, steam drum level, and feed water flow, and operates on an instantaneous basis as follows: When a change of feed water flow is desired by the boiler controls, it sends a signal to the scoop tube positioner, which moves the scoop tube, which changes the inside radius of the oil ring, which changes the output shaft speed. This method provides a very quick and powerful change of speed. The problem that occurs with higher circuit oil flow is that it becomes more difficult to control the output shaft speed, hence feed water flow, at lower feed water flow rates, and for this reason lower circuit oil flow is desirable. Consequently, determining the desired constant flow rate for the circuit oil can become a serious point of contention, particularly in times of hot cooling water due to hot ambient conditions, such as on a hot summer day.
There is another reason to minimize the circuit oil flow through the impeller/runner/casing/scoop tube. As indicated in Tables 2 and 3, several hundred horsepower are required to drive the oil into the impeller and out through the scoop tube. This power loss is proportional to the radial position of the scoop tube and is directly proportional to the circuit oil flow rate. With operating conditions that cause reduced heat loss, the circuit oil flow can be reduced, providing improved overall operating efficiency of the fluid drive, as indicated by the column HP SAVED in Table 3. This represents fuel that does not have to be burned to provide the same net electrical generation to customers, and therefore is an environmental improvement.
The circuit oil discharge temperature is measured by thermo-couples or similar devices located in a collection tray mounted to the inside wall of the fluid drive tank, so that these thermocouples can provide information to controllers, recording devices and/or monitors remote from the fluid drive. If an excessive temperature is detected, as would occur in the case of "freezing" of the boiler feedpump due to galling between the rotating pump element and the stationary components as would occur if the boiler feed water supply were interrupted, overfilling a fluid drive, or loss of cooling water to the oil heat exchangers, etc., then the fluid drive, circuit oil pumps, and turbine-generator are shut down automatically.
When the fluid drive is shut down for any reason, normal or abnormal, it still requires lube oil and should have circuit cooling oil. This is particularly true for a fluid drive that is attached to a turbine-generator, as these often require 35 to 45 minutes for the rotational speed to drop from 3600 rpm to zero.
The following describes the operational characteristics of the three aforementioned conventional oil systems during the shut-down period, both with and without the availability of A.C. power.